Such control circuits are known. An input signal of the control circuit is usually routed to a summing point, whose output leads into the control path that emits the output signal of the control circuit. The output signal is measured and coupled back with a negative sign to the feedback input of the summing point, i.e., the input of the control circuit. This makes it possible to control the output signal of the control circuit as a function of its input signal in compliance with the requirements and control characteristics pertaining to the control circuit.
The output signal of the control circuit is measured by means of a sensor selected based on the variable to be physically acquired. This sensor can be arranged either in the control path itself or in the feedback path. The control circuit is closed via the feedback signal, which forms in the feedback path, and is superimposed with a negative sign on the input signal.
Sensors for physical variables, e.g., magnetic or electric fields, or for mechanical or chemical variables, exhibit a sensor-typical characteristic for the output signal as a function of the input variable to be measured. These characteristics are often linear only in a small region, the operating range of the sensor, and exhibit a nonlinear characteristic outside this region between the input variable to be measured and the output signal of the sensor. This results in nonlinearities that must be specially considered in a control circuit, and are often difficult to correct.
In certain instances, the characteristic of a sensor proceeds in a nonlinear manner, wherein the sensitivity, i.e., the output signal of the sensor, no longer increases as the measuring variable rises, but the sensitivity tapers off again after a maximum has been exceeded as the measuring variable continues to rise. Sensitivity follows the opposite pattern by increasing again after falling below a minimum as the measuring variable drops. This behavior is observed, for example, in the HNC 1001/1002 magnetoresistive sensor made by Honeywell. At a magnetic field of 0 Oe, this sensor exhibits virtually no output voltage. In a range of up to 5 Oe, there is a linear dependence between the magnetic field and output voltage. Sensitivity tapers off as the magnetic field continues to rise, i.e., the output voltage of the sensor no longer rises to the same extent. At roughly 11 Oe, the sensitivity curve peaks. At higher magnetic flux densities, the output voltage of the sensor tapers off toward 0. Negative flux densities of the magnetic field produce a mirror image curve.
In a general controller, in particular in devices with magnetoresistive sensors, the nonlinearities caused by the sensitivity curve of the sensor negatively impact the properties of the closed control circuit. In overload situations involving a sharp rise in the input variable, the signal output of the sensor might not emit a higher signal, but instead a lower signal not corresponding to the input signal to be measured. This means that the negative feedback loop might not be able to correct the control circuit any longer in these extreme operating situations. Such an effect is referred to as foldback, and is undesired in all applications.